The present invention relates to a spark plug for use on internal combustion engines, more particularly to one having a built-in resistor for preventing the occurrence of electrical noise.
A conventional spark plug with a built-in resistor that is of the type contemplated by the invention comprises an insulator having an axially extending passing-through-hole, a terminal metal fitting inserted into the passing-through-hole from one end and fixed therein, a center electrode inserted into the same passing-through-hole from the other end and fixed therein, and a resistor provided between the terminal metal fitting and the center electrode within the passing-through-hole. The effectiveness of such spark plug in preventing electrical noise will generally improve as the length of the resistor increases.
With the conventional spark plug having a built-in resistor, it has been essential that a sealing layer of electroconductive glass be interposed between the resistor and each of the terminal metal fitting and the center electrode in order to insure that the respective elements have positive electrical joint. As a result, the length of the resistor inevitably decreases by an amount that corresponds to the required thickness of the conductive glass seal layers provided in the space where the terminal metal fitting faces the center electrode. Hence, given a limited space where the terminal metal fitting faces the center electrode, it has been impossible to increase the length of the resistor sufficiently to realize a marked improvement in the prevention of electrical noise.
An object, therefore, of the present invention is to provide a spark plug with a built-in resistor in an insulator that allows for an increase in the length of the resistor even if the outer dimensions of the insulator are limited and which thereby assures more effective prevention of electrical noise.